1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly to an organic electroluminescent device with an improved electrode structure.
2. Discussion of the Related Art
Generally, electroluminescent (EL) devices are self emission displays that emit light by electrical excitation of fluorescent organic compounds, and they have many advantages over liquid crystal displays (LCDs), including a lower driving voltage, a thinner depth, a wider viewing angle, and a faster response speed. Therefore, there has been increasing interest in developing EL devices as next generation displays.
EL devices are either inorganic or organic, depending upon whether a light-emitting layer is made of an inorganic material or an organic material.
With organic EL devices, an organic layer is formed in a predetermined pattern on a glass or a transparent insulating substrate, and electrode layers are formed on the organic layer's upper surface and lower surface. Typical organic compounds used for the organic layer include a phthalocyanine, such as copper phthalocyanine (CuPc), N,N-di(naphthalene-1-yl)-N,N′-diphenyl-bezidine (NPB), or tris-8-hydroxyquinoline aluminum (Alq3).
When positive and negative voltages are applied to electrodes of the above-described organic EL devices, holes from the electrode connected to the positive voltage migrate toward a light-emitting layer via a hole transport layer, and electrons from the electrode connected to the negative voltage are injected into the light-emitting layer via an electron transport layer. The electrons and holes then recombine in the light-emitting layer to generate excitons. When the excitons change from an excited state to a ground state, fluorescent molecules of the light-emitting layer-emit light, which displays an image.
Light emitted from the light-emitting layer may be discharged through the top of the substrate. It may also be emitted through a sealing member at the bottom of the substrate, or it may be emitted in both directions. The former EL device is designated as a front emission type, and the latter is designated as a both-direction emission type.
FIG. 1 shows an organic EL device having a structure that may be used as a front emission type and a both-direction emission type. Referring to FIG. 1, an organic EL device includes a glass substrate 11, a first electrode layer 12 formed on the glass substrate 11, an organic layer 13, which may have a multilayer structure, formed on the first electrode layer 12, a second electrode layer 14, which may be made of a metal, formed on the organic layer 13, and a transparent auxiliary electrode layer 15 formed on the second electrode layer 14.
The first electrode layer 12 may be made of a transparent material, such as Indium Tin Oxide(ITO) or Indium Zinc Oxide(IZO), or a reflective material, such as metal. The second electrode layer 14 may be a metallic thin film. The auxiliary electrode layer 15 may be made of a transparent conductive material, such as ITO or IZO, to reinforce the conductivity of the second electrode layer 14. Therefore, light emitted from a light-emitting layer (not shown) in the organic layer 13 may be discharged through the second electrode layer 14 or through both the second electrode layer 14 and the substrate 11 to create an image.
However, in an organic EL device with the above-described structure, in order to enhance light transmittance, the second electrode layer 14 may be formed at a depth of about 100 Å. But because the layer is thin, a lack of film uniformity may occur, which may generate pinholes 14a as shown in FIG. 1. These pinholes 14a may allow defects to be formed in the EL device when the transparent auxiliary electrode layer 15 is deposited on the second electrode layer 14.
In other words, if the auxiliary electrode layer 15 is deposited by sputtering ITO or IZO, the organic layer 13, which is exposed through the pinholes 14a, may be damaged by plasma or energetic particles. This phenomenon worsens as the second electrode layer 14 decreases in thickness, because as the second electrode layer 14 becomes thinner, its uniformity worsens. As a result, the second electrode layer 14 may be distributed in an island shape, where it may insufficiently cover the organic layer 13.
When plasma or energetic particles damage the organic layer 13, its damaged portions D may act as dark spots.
In view of these problems, the second electrode layer 14 may be formed thicker, but this may lower light transmittance, thereby decreasing luminance efficiency. Furthermore, high power consumption is required to compensate for this decreased luminance efficiency.
U.S. Pat. No. 6,284,393 discloses an organic EL device including a conductive film sandwiched between a cathode and an organic layer. However, this patent is silent about problems caused by a thin cathode.